Method for the synthesis of radiolabeled compounds

ABSTRACT

The present invention relates to a new method for radiolabeling chemical compounds. The new method attains the goals of simplicity, high radiochemical yields, speed, versatility, and automation. An HPLC injection loop on an HPLC injection valve is loaded with a solution of precursor and the radiolabeling reagent is passed through the loop. The contents of the loop are then quantitatively injected onto the HPLC column for purification. Radiochemical yields are equal to or superior to conventional solution methods in all cases, even though no heat need be applied. Since no vials, transfer lines, cooling, heating, or sealing valves are required, no transfer losses occur, yields are high, and clean-up is minimal. This “loop method” is ideal for the preparation of radiolabeled compounds, in particular those prepared from [ 11 C]-iodomethane.

[0001] This application claims the benefit under 35 USC §119(e) fromU.S. provisional patent application Ser. No. 60/265,126, filed Jan. 31,2001.

FIELD OF THE INVENTION

[0002] The invention relates to a method for radiolabeling chemicalcompounds. Specifically, the method of the invention provides a simpleand efficient method to [¹¹C]-methylate target substrates.

BACKGROUND OF THE INVENTION

[0003] Methylation of suitable precursors with [¹¹C]-iodomethanecontinues to be the workhorse method of synthesis of a majority ofpositron emission tomography (PET) radiotracers, especially in the areaof receptor imaging. Many improvements have been made in the productionof [¹¹C]-iodomethane, most recently with the introduction of “gas-phasemethods” which are highly automated and produce a product with very highspecific activity (1,9,10). Efforts to simply and automate the[¹¹C]-methylation reaction and purification have been less spectacular.Many attempts to streamline the process have revolved around the idea ofeliminating the traditional “solution in reaction vial” whereby[¹¹C]-iodomethane is distilled into a vessel containing solvent,precursor, and base/catalyst if required (1,8.11). In this commonscenario, cooling of the vessel for trapping [¹¹C]-iodomethane isusually required followed by sealing of the vessel, heating to effectreaction, quenching of the reaction, and transferring of the vesselcontents to a high performance liquid chromatography (HPLC) system forpurification. The streamlining and automation of this sequence ofevents, especially the last, has proven onerous.

[0004] An attractive technique to surmount some of these problems wasdescribed in the pioneering papers of Jewett and Watkins on captivesolvent chemistry (4,6,17). Their goal was to develop a solid support totrap reagents and iodomethane together, eliminating the need for areaction vessel with its septa, needles, and their associated problems.Similar efforts have been forthcoming from other groups where thetechnique has been named on-line, solid-phase, or immobilized techniques(3,14,16,19). When the solid support is plumbed in to take the place ofthe HPLC sample loading loop then transfer losses from reaction vesselto loop can also be eliminated and the process simplified.

[0005] Such captive solvent techniques have not been widely adopted bythe field, perhaps because no one method fulfills all the requirementsof ease of use reproducibility, and versatility. Some methods have notbeen fully integrated as part of the HPLC system (17,19), some requirenon-proprietary solid-supports (6,14), while others can degrade the HPLCpurification of the radiotracer (3). In addition, cooling and heatingthe reaction site are still necessary steps in the synthetic sequence.

[0006] There remains a need for a simple, fast and versatile method toradiolabel chemical compounds, specifically via [¹¹C]-methylationreactions, which provides high radiochemical yields and is easilyautomated.

SUMMARY OF THE INVENTION

[0007] A new method to radiolabel chemical compounds has been developed.Specifically the method involves the [¹¹C]-methylation of chemicalcompounds. In general terms, the method involves trapping aradiolabeling reagent directly in a standard HPLC loop coated withprecursor solution, allowing the reagents to react, and then directlyinjecting the reaction mixture onto an HPLC purification column. Themethod does not require the use of any additional solid support nor anyheating or cooling.

[0008] The present invention therefore provides a method forradiolabeling precursor chemical compounds comprising the steps of:

[0009] injecting a sample comprising a precursor chemical compound intoan injection loop of a high performance liquid chromatograph (HPLC);

[0010] injecting a radiolabeling reagent into the injection loop;

[0011] allowing the radiolabeling reagent to react with the precursorchemical compound, to provide a reaction mixture comprising aradiolabeled compound;

[0012] injecting the reaction mixture into the HPLC column; and

[0013] isolating the radiolabeled compound.

[0014] In embodiments of the present invention, the radiolabelingreagent is a [¹¹C]-methylating reagent. Accordingly, the presentinvention further provides a method for [¹¹C]-methylation of precursorchemical compounds comprising the steps of:

[0015] injecting a sample comprising a precursor chemical compound intoan injection loop of a high performance liquid chromatograph (HPLC);

[0016] passing [¹¹C]-iodomethane through the injection;

[0017] allowing the [¹¹C]-iodomethane to react with the precursorchemical compound, to provide a reaction mixture comprising a[¹¹C]-methylated compound;

[0018] injecting the reaction mixture into the HPLC column; and

[0019] isolating the [¹¹C]-methylated compound.

[0020] Some literature methods for performing [¹¹C]-methylationreactions involve the use of a solid support as the means to bringtogether precursor and [¹¹C]-iodomethane. A disadvantage of this is thatthe reaction mixture is strongly retained by the support with the resultthat HPLC separations are less efficient (3). Using only the loop as asupport avoids this pitfall and no differences in chromatographicseparations could be discerned as compared to injections of solutions ofreaction mixtures.

[0021] Other features and advantages of the present invention willbecome apparent from the following detailed description. It should beunderstood, however, that the detailed description and the specificexamples while indicating preferred embodiments of the invention aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention will now be described in relation to the drawingsin which:

[0023]FIG. 1 is a schematic of an exemplary system and flow diagram fortrapping a radiolabeling reagent in an HPLC loop coated with precursorsolution, for use in the method of the invention.

[0024]FIG. 2 is a schematic of plumbing used in an exemplary automatedsystem for the radiolabeling reactions of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0025] (I) Method of the Invention

[0026] The present inventors have developed a new method to radiolabelchemical compounds. Specifically the method involves the[¹¹C]-methylation of chemical compounds. In general terms the methodinvolves trapping a radiolabeling reagent directly in a standard HPLCloop coated with precursor solution, allowing the reagents to react, andthen directly injecting the reaction mixture onto a HPLC purificationcolumn. The method does not require the use of any additional solidsupport nor any heating or cooling.

[0027] The present invention therefore provides a method forradiolabeling precursor chemical compounds comprising the steps of:

[0028] injecting a sample comprising a precursor chemical compound intoan injection loop of a high performance liquid chromatograph (HPLC);

[0029] injecting a radiolabeling reagent into the injection loop;

[0030] allowing the radiolabeling reagent to react with the precursorchemical compound, to provide a reaction mixture comprising aradiolabeled compound;

[0031] injecting the reaction mixture into the HPLC column; and

[0032] isolating the radiolabeled compound.

[0033] The radiolabeling reagent may be any such compound that works inthe method of the invention. Suitably, the radiolabeling reagent is avolatile and condensable compound. Other radiolabeling reagents that arenot volatile or condensable, may be used by first dissolving in anappropriate solvent and injecting the resulting solution into a standardHPLC loop coated with precursor solution. In an embodiment of thepresent invention, the radiolabeling reagent is an alkylating reagent.For example, the radiolabeling may be selected from the group consistingof [¹¹C]-ethyl iodide, [¹¹C]-propyl iodide, [¹¹C]-methyliodide and[¹¹C]-acetyl iodide.

[0034] Preferably, the radiolabeling reagent is [¹¹C]-methyliodide.Accordingly, the present invention further provides a method for the[¹¹C]-methylation of precursor chemical compounds comprising the stepsof:

[0035] injecting a sample comprising a precursor chemical compound intoan injection loop of a high performance liquid chromatograph (HPLC);

[0036] passing [¹¹C]-iodomethane through the injection loop;

[0037] allowing the [¹¹C]-iodomethane to react with the precursorchemical compound, to provide a reaction mixture comprising a[¹¹C]-methylated compound;

[0038] injecting the reaction mixture into the HPLC column; and

[0039] isolating the [¹¹C]-methylated compound.

[0040] Any precursor compound known to react in standard radiolabelingreactions, specifically [¹¹C]-methylation reactions, may be used in themethod of the invention. Preferably the precursor chemical compound isdissolved in a suitable solvent A person skilled in the art would beable to select a suitable solvent for dissolving the precursor chemicalcompound based on the physical properties of this compound.

[0041] The selection of HPLC column type will be dependent on the choiceof solvent and the identity of the precursor chemical compound and wouldbe apparent to those skilled in the art. In some cases, a base and/or acatalyst may be included in the initial solution of precursor chemicalcompound. Suitable bases include inorganic bases such as sodiumbicarbonate, sodium hydroxide and the like. Bases may be required whenthe precursor compound is in an acid form. Suitable catalysts includequaternary ammonium salts and the like.

[0042] Standard HPLC injection valves, loops and columns may be usedwith the method of the invention. An example of a standard set-up isshown in FIG. 1. The volume of solution of precursor (and base/catalyst,if necessary) applied to the loop should be designed so that the entireloop surface is coated; larger volumes may result in breakthrough of thesolvent upon purging the loop with N₂ gas. [¹¹C]-iodomethane (or othervolatile condensable labeling reagents) may be added to the HPLC loopcoated with the solution of precursor (and base/catalyst if necessary),in a stream of nitrogen, preferably until the level of radioactivitytrapped on the loop reaches a maximum. The level of radioactivity may bedetected, for example, using a proximal radiation detector in the HPLCloop. In the case of [¹¹C]-iodomethane, and other iodinatedradiolabeling reagents, a small photoiodide radiation detectorpositioned close to the sample loop provides the operator withinformation on both the amount and trapping of iodinated radiolabelinside the sample loop. The amount of time required to reach a maximumlevel of radioactivity in the loop will depend on the size of the loop.Once this maximum activity level is reached, the flow of nitrogen may beturned off and the chemicals allowed to react until the reaction isdeemed complete, for example, about 0.5 to about 20 minutes, preferablyfrom about 1 to about 10 minutes, most preferably from about 1 to about5 minutes. Alternately, an aliquot of a radiolabeling reagent dissolvedin an appropriate solvent may be injected into the HPLC loop coated withprecursor compound (and base/catalyst if necessary) and the reagentsallowed to react as described above. The contents of the loop may theninjected into the HPLC column and the resulting products purified andisolated using standard procedures. The loop should be cleaned prior touse for the method of the invention. This is simply done by passingsolvent, for example methanol, acetone or water (depending on thecompounds used in a previous reaction), through the loop and blowingdry.

[0043] The method of the invention has been applied to the synthesis ofa variety of PET pharmaceuticals (Table 1). In all cases the method wassuccessful with results equal to or superior to those using conventionalsolutions in a vial methods. This includes such parameters of successsuch as trapping of [¹¹C]-iodomethane, radiochemical yield, time ofsynthesis, purity of final product, and specific activity. In additionto the compounds listed in Table 1 the method has been successfullyapplied to the [¹¹C]-methylation of the thiol group of (R)-(+)McNeil5652 and the N1 of 6-chloro-1H-benzoimidazole-4-carboxylic acid(1-isobutyl-piperidin-4-ylmethyl)-amide.

[0044] In the method of the invention, [¹¹C]-iodomethane is trappedefficiently without cooling or the use of a solid support to increasesurface area, and the methylations occur quickly, without the aid ofheat. Trapping in the loop is not simply a consequence of reaction of[¹¹C]-iodomethane with precursor. Experiments showed that loading theloop with DMF (70 μL) with no added base or precursor also resultedin >90% trapping efficiency of the [¹¹C]-iodomethane.

[0045] The rate of reaction of [¹¹C]-iodomethane with precursorcompounds at ambient temperature is fast. This compares with literaturereports or other methods requiring heating, often to 90° C. for 5 min(12). Under the conditions described in Table 1, unreacted[¹¹C]-iodomethane accounted for less than 10% of total activity by HPLCi.e. reactions were at least 90% complete.

[0046] The versatility of the method is apparent upon examination of thevariety of radiotracers synthesized (Table 1). A number of functionalgroups have been methylated with [¹¹C]-iodomethane using this methodincluding phenols, acids, thiols, amides, and secondary amines. Since novials, transfer lines, cooling, heating, or sealing valves are required,no transfer losses occur, yields are high, and clean-up is minimal, thismethod is ideal for most radiotracers prepared using [¹¹C]-iodomethane.

[0047] (II) Apparatus

[0048] As hereinbefore mentioned, standard HPLC injection valves, loopsand columns may be used with the method of the invention. An example ofa standard set-up is shown in FIG. 1. FIG. 2 shows a schematic of asystem for use with the method of the invention wherein three othervalves (in addition to the original HPLC injection valve) have beenadded to allow automatic cleaning of the sample loop upon completion ofthe reaction. A solvent select valve allows for the sequential passageof cleaning solvent and drying gas through the loop. A small diaphragmvacuum pump may be used to suck the cleaning solvents through thesystem, but pressure drive solvents (either by gas or pump) will work aswell.

[0049] The following non-limiting examples are illustrative of thepresent invention:

EXAMPLES Methods

[0050] Precursors (normethyl) for [¹¹C]-methylation were synthesized inhouse (RTI-32, rolipram, FLB 457, DASB), or were commercially available(SCH 23390, NMS, SKF 82957). Raclopride precursor was a gift from AstraArcus AB (Sodertaije, Sweden) and Ro 15-1788 precursor was a gift fromHoffman-LaRoche (New Jersey, USA).

General Method for Performing [¹¹C]-methylations in an HPLC InjectionLoop

[0051] (a) Set Up

[0052] A commercial HPLC injection valve (Valco, Tex., USA, #AC6W) wasequipped with a commercial stainless steel loop (2 mL, Valco #SL2KCW)and a commercial injection port (Valco #VISF-2). A charcoal trap wasconnected to the waste port of the HPLC valve and placed in a wellcounter to measure untapped [¹¹C]-iodomethane (FIG. 1). With the valvein the LOAD position, a solution of precursor, solvent, and base (ifrequired, total volume (80 μL) was injected slowly (5- 10 sec) into theclean, dry injection loop using a 100 μL gas-tight syringe (Hamilton,Nev., USA, #1810). The injection port was replaced by the[¹¹C]-iodomethane transfer line which had a conditioning flow of 8mL/min N₂ gas passing through it. The loading of the loop with precursorsolution could be made at least up to 10 min before end-of-bombardment(EOB) and no solvent emerges from the waste port. See Table 1 forspecific reaction conditions.

[0053] (b) Trapping and Reaction of [¹¹C]-iodomethane

[0054] [¹¹C]-iodomethane (produced as previously described (18)) wasswept into the HPLC loop coated with precursor solution by a stream ofN₂ gas (8 mL/min) at ambient temperature. Radioactivity trapped on theloop was detected by a proximal radiation detector. When activity peakedin the loop (3-4 min), the flow of N₂ was stopped and the reactionallowed to proceed (1-5 min). The contents of the loop were thenquantitatively injected onto the HPLC purification column by simplychanging the position of the injection valve to INJECT.

[0055] (c) Cleanup

[0056] The loop had already been cleaned by the HPLC eluent. Preparationfor its next run simply involved switching the valve back to the LOADposition, passing a few mL of ethanol (or water if mobile phase containsphosphate buffer) and acetone through the loop, and blowing dry.

Preparation of Raclopride Precursor (Free Base)

[0057] Raclopride precursor (HBr salt, 25.0 mg,) was dissolved in 2 mLof methanol and 1 mL of water. Aqueous sodium bicarbonate (1 N, 1.5 mL)was added and the clear solution was evaporated under vacuum to removethe methanol (no heating). The off-white precipitate was collected byfiltration under vacuum and dried in a vacuum desiccator (17.87 mg, 89%yield).

[0058] The [¹¹C]-methylation method of the invention has been applied tothe synthesis of a variety of PET radiopharmaceuticals (Table 1). Inaddition to the compounds listed in Table 1, the method has beensuccessfully applied to the [¹¹C]-methylation of the thiol group of(R)-(+)McNeil 5652 and the N1 of 6-chloro-1H-benzoimidazole4carboxylicacid (1-isobutyl-piperidin4-ylmethyl)-amide.

[0059] The sequence of events and the state of the different componentsof the system for the method of the invention wherein the cleaningcycles are automated are detailed in Table 2.

Procedure

[0060] 1) Stand-bye: start position in which all valves/devices arepowered off or in default position. If [¹¹C]CH₃I module is beingconditioned by slow nitrogen flow, then so is the HPLC sample loop.Precursor solution injected manually onto HPLC loop before synthesis.This is the only step that requires operator intervention in the hotcell.

[0061] 2) Trapping: no change in valve settings but radiation detectorand timer employed. Initiated upon start of [¹¹C]CH₃I distillation.

[0062] 3) Reaction: no change in valve settings, simply a preset waitingtime interval for the methylation reaction to proceed. Initiated eitherby operator or by detection of plateau of trapped radioactivity levelsin HPLC loop.

[0063] 4) Purification: reaction mixture injected onto HPLC column bysimply switching position of HPLC valve from position A to Position B.Initiated by timer count down.

[0064] 5) Clean 1: HPLC loop cleaned by sucking water through it for 60sec. Settings are shown in the Table above. Initiated anytime afterreaction mixture has been washed onto HPLC column.

[0065] 6) Clean 2: HPLC loop cleaned by sucking acetone through it for60 sec. Condition follows Clean 1 condition automatically.

[0066] 7) Dry: HPLC loop dried by blowing nitrogen gas (10 psi) throughit for 5 min. Condition follows Clean 2 condition automatically.

[0067] 8) Stand-bye: (exit condition)

Notes

[0068] a) Washing and drying times are first guess values. They workwell, but shorter times may be possible if desired.

[0069] b) Only about 10 cc of wash solvents are used per run. As thesolvent reservoirs hold 500 ml they need to be replenished after about50 clean cycles.

[0070] c) A program written in Labview 5.1.1 for Windows (NationalInstruments, Texas) controls the module.

[0071] d) All components are commercially available except thephotodiode radiation detector, which was built in house.

[0072] A person skilled in the art would know how to modify the aboveconditions so that radiolabeling reagents other than [¹¹C]-methyliodidecould be used.

[0073] While the present invention has been described with reference towhat are presently considered to be the preferred examples, it is to beunderstood that the invention is not limited to the disclosed examples.To the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

[0074] All publications, patents and patent applications are hereinincorporated by reference in their entirety to the same extent as ifeach individual publication, patent or patent application wasspecifically and individually indicated to be incorporated by referencein its entirety.

Full Citations for References Referred to in the Specification

[0075] 1. Crouzel, C., Langstrom, B., Pike, V. W., and Coenen, H. H.(1987) Recommendations for a practical production of [¹¹C]methyl iodideAppl. Radiat Isot Int. J. Appl. Instrum. Part A 38, 601-603.

[0076] 2. Dannals, R. F., Ravert, H. T., and Wilson, A. A. (1990)Radiochemistry of Tracers for Neurotransmitter Receptor Studies. In:Quantitative Imaging: Neuroreceptors, Neurotransmitters, and Enzymes.(Edited by Frost, J. J. Wagner Jr., H. N.) pp. 19-35. Raven Press, NewYork

[0077] 3. Iwata, R., Pascali, C., Yuasa, M., Yanai, K., Takahashi, T.,and Ido, T. (1992) On-line [¹¹C]Methylation using [¹¹C]Methyl Iodide forthe Automated Preparation of ¹¹C-Radopharmaceuticals Appl. Radiat Isot.43, 1083-1088.

[0078] 4. Jewett, D., Ehrenkaufer, R., and Ram, S. (1985) A CaptiveSolvent Method for Rapid Radiosynthesis: Application to the Synthesis of[1-¹¹C]Palmitic Acid Int. J. Appl. Radiat. Isot. 36, 672-674.

[0079] 5. Jewett, D. M. (1992) A Simple Synthesis of [¹¹C]MethylTriflate Appl. Radiat. Isot. 43, 1383-1385.

[0080] 6. Jewett, D. M., Manger, T. J., and Watkins, G. L. (1991)Captive Solvent Methods for Fast Simple Carbon-11 Radioalkylations. In:New Trends in Radiopharmaceutical Synthesis, Quality Assurance andRegulatory Control (Edited by Emran, A. M.) pp. 387-391. Plenum Press,New York

[0081] 7. Langer, O., Nagren, K., Dolle, F., Lundkvist, C., Sandell, J.,Swahn, C.-G., Vaufrey. F., Crouzel, C., Maziere, B., and Halidin, C.(1999) Precursor Synthesis and Radiolabeling of the Dopamine D2 ReceptorLigand [11 C}Raclopride from [¹¹C]Methyl Triflate. J. Labelled Compd.Radiopharm. 42, 1183-1193.

[0082] 8. Längström, B., and Lundqvist, H. (1976) The preparation of¹¹C-methyl iodide and its use in the synthesis of11C-methyl-L-methionine Appl. Radiat. Isol. 27, 357-363.

[0083] 9. Larsen, P., Ulin, J., Dahistrom, K., and Jensen, M. (1997)Synthesis of [¹¹C]Iodomethane by Iodination of [¹¹C]Methane Appl.Radiat. Isot 48, 153-157.

[0084] 10. Unk, J. M., Krohn, K. A., and Clark, J. C. (1997) Productionof [¹¹C]CH₃I by single pass reaction of [¹¹C]CH₄ with I₂ Nucl Med Biol24, 93-97.

[0085] 11. Marazano, C., Maziere, M., Berger, G., and Comar, D. (1977)Synthesis of methyl iodide-¹¹C and formaldehyde-¹¹C Appl. Radiat Isot28, 49-52.

[0086] 12. Mazière, B., Coenen, H., Halldin, C., Nagren, K., and Pike,V. (1992) PET radioligands for dopamine receptors and re-uptake sites:chemistry and biochemistry Nuc. Med. Biol. 19,497-512.

[0087] 13. McCarron, J. A., Turton, D. R., Pike, V. W., and Poole, K. G.(1996) Remotely-controlled production of the 5-HT_(1A) receptorradioligand, [carbonyl-C-11]WAY-100635, via C-11-carboxylation of animmobilized Grignard reagent J Labelled Compds. Radiopharm. 38, 941-953.

[0088] 14. Mizuno, K. I., Yamazaki, S., Iwata, R., Pascali, C., and Ido,T. (1993) Improved Preparation of L-[Methyl-C-11]Methionine by on-Line[C-11]Methylation Applied Radiation and Isotopes 44, 788-790.

[0089] 15. Parker, A. (1969) Protic-Dipolar Aprotic Solvent Effects onRates of Bimolecular Reactions Chem. Rev. 69, 1-33.

[0090] 16. Pascali, C., Iwata, R., and Ido, T. (1992) Comparative Studyon the Influence of Bases on (3-N-[C-11]Methyl)Spiperone Synthesis Int.J. Appl. Radiat. Isot. 43,1526-1528.

[0091] 17. Watkins, G., Jewett, D., Mulholland, G., Kilbourn, M., andToorongian, S. (1988) A Captive Solvent Method for RapidN-[¹¹C]Methylation of Secondary Amides: Application to theBenzodiazepine, 4′-Chlorodiazepam (RO5-4864) Appl. Radiat. Isot. 39,441-444.

[0092] 18. Wilson, A. A., DaSilva, J. N., and Houle, S. (1996) In vivoevaluation of (11 C] and [¹⁸F]-labelled cocaine analogues as potentialdopamine transporter ligands for positron emission tomography Nuc. Med.Biol. 23, 141-146.

[0093] 19. Wilson, A. A., DaSilva, J. N., and Houle, S. (1996)Solid-phase radiosynthesis of [¹¹C]WAY 100635 J. Labelled Compds.Radiopharm. 38, 149-154. TABLE 1 Results of using the method of theinvention for the radiosynthesis of a variety of radiotracers^(a).Reaction Yield Specific Activity Base Time precursor Compound (GBq)^(b)(GBq/μmole) (μL) (min) (mg) RTI-32 11.1 89 4^(c) 1 0.9 SCH 23390 8.4 5110^(d)  5 1.0^(e) FLB 457 4.4 97 2^(f) 5 1.2 NMS^(g) 10.7 93  2.5^(f) 21.0 Raclopride 5.1 48 3^(h) 5 1.1¹ Ro 15-1788 13.8 98 4^(c) 2 0.6Rolipram 8.1 81 3^(j) 3 0.3 SKF 82957 8.1 84 10^(d)  5 1.0^(k) DASB^(l)8.7 57 n/a 5 1.0

[0094] TABLE 2 Event sequence for [¹¹C]-methylations inside an HPLCsample loop HPLC Solvent Select Valves Valve Radiation Vacuum ConditionValve Position A & B Position Detector Pump Stand-bye Vent off PositionA Off Off Trapping Vent off Position A On Off Reaction Vent off PositionA On Off Purification Vent off Position B On Off Clean 1 water onPosition A Off On Clean 2 acetone on Position A Off On Dry nitrogen onPosition A Off Off

We claim:
 1. A method for radiolabeling precursor chemical compoundscomprising the steps of: injecting a sample comprising a precursorchemical compound, into an injection loop of a high performance liquidchromatograph (HPLC); injecting a radiolabeling reagent into theinjection loop; allowing the radiolabeling reagent to react with theprecursor chemical compound, to provide a reaction mixture comprising aradiolabeled compound; injecting the reaction mixture into the HPLCcolumn; and isolating the radiolabeled compound.
 2. The method accordingto claim 1, wherein the radiolabeling reagent is a volatile andcondensable compound.
 3. The method according to claim 2, wherein theradiolabeling reagent is selected from the group consisting of[¹¹C]-ethyl iodide, [¹¹C]-propyl iodide, [¹¹C]-methyliodide and[¹¹C]-acetyl iodide.
 4. The method according to claim 3, wherein theradiolabeling reagent is [¹¹C]-methyliodide.
 5. The method according toclaim 1, wherein the precursor chemical compound is in the form of anacid salt and the sample further comprises a base.
 6. The methodaccording to claim 4, wherein the [¹¹C]-iodomethane is reacted with theprecursor chemical compound for about 0.5 to about 20 minutes.
 7. Themethod according to a claim 1, wherein the precursor chemical compoundis dissolved in a solvent.
 8. The method according to claim 1, whereinthe sample further comprises a catalyst.
 9. A radiolabeled compoundprepared using a method according to claim
 1. 10. A [¹¹C]-methylatedcompound prepared using a method according to claim 4.